1. Field of the Invention
The present invention relates to an optical scanning device and an image forming device, and more particularly, to an optical scanning device in which a light beam scans a scanning target surface and an image forming device having such an optical scanning device.
2. Description of the Related Art
In the related art, an optical scanning device is widely used in image forming devices such as an optical printer, a digital copying machine, and an optical plotter. Concomitantly with the popularization of the image forming devices, it is strongly required for the image forming devices to be high in stability, resolution, and operation speed and to be realized with a reduced number of components in a reduced size.
Recently, a heterochromatic image forming device has been developed in which a plurality of color images are superimposed. Particularly, for realization of the heterochromatic image forming device, a tandem type is widely used in which a plurality of scanning optical systems form light beam spots on a plurality of photosensitive elements, each corresponding to one color.
For the writing of the heterochromatic images, provided is a method is using a plurality of optical scanning devices, each corresponding to one color. However, this method is disadvantageous in that the number of components increases because optical components are provided for each color, and miniaturization is difficult because a plurality of optical scanning devices are mounted in the image forming device.
In this regard, proposed is a method of using a single optical scanning device with a plurality of scanning optical systems. In this method, generally, a plurality of light beams, each corresponding to one color, are incident onto a single optical deflector and are focused on each photosensitive element using each scanning optical system (refer to FIG. 46).
However, in this method, optical elements for each color are concentrically disposed near the optical deflector so that miniaturization of the optical scanning device is limited.
In order to overcome such a limitation in miniaturizing the optical scanning device due to the multiple colors, two methods have been proposed as follows.
As a first method, two vertically-overlapping scanning optical systems are integrated by changing polarization directions of each light beam, and the light beams are split using a polarized light beam splitting device (polarization splitting element) provided in the scanning optical system, so that each light beam is guided to one photosensitive element (herein, referred to as a “polarization splitting method”).
In this method, the size of the optical scanning device is reduced in a rotation axis direction of the optical deflector (to be thinner), and optical components in the scanning optical system are shared between the vertically overlapped stages, so that the number of components is effectively reduced.
As a second method, a plurality of scanning optical systems are integrated by slightly separating optical paths of each light beam, and optical paths are separated by organizing an polarization-independent half mirror, so that each light beam is guided to one photosensitive element (herein, referred to as a polarization-independent splitting method).
In addition, other methods have been considered, including a method of integrating scanning optical systems using a dynamic active element such as a spatial modulation element or a method of splitting light beams at a dichroic mirror using light sources radiating different wavelengths. However, when the active element is used, a driving circuit is required. In addition, when the dichroic mirror is used, different types of light sources are mounted on a single optical scanning device. Therefore, even when the number of optical elements is reduced in the scanning optical system, it is necessary to further increase the number of expensive elements as a trade-off.
In this regard, the polarization splitting method described above or a polarization-independent splitting method is preferably used.
When the polarization splitting method is used, it is necessary to improve a splitting property of the polarization splitting element. For example, if the light beams incident to the polarization splitting element are slightly elliptically polarized, or polarization directions thereof are inclined, the light beams are mixed in the other scanning optical system although they are to be split from each other.
The respective light beams are radiated as different time series signals in order to write image information onto each scanning target surface. In this regard, if the polarization splitting property is not appropriate, image information to be written to other scanning target surfaces may be mixedly incorporated. For example, in the heterochromatic image forming device, information to be developed as cyan may be written to the scanning target surface for magenta. This is observed as a crosstalk between colors on the output image.
The main factor of degrading the polarization splitting property is birefringence caused when resin is used in the scanning lens. Although a resin material having a low birefringence rate is widely studied in the art, its application has some problems to be solved in consideration of the shape of the scanning lens, molding conditions, and manufacturing efficiency. Although a method of organizing the entire scanning optical system with glass lenses in order to avoid the birefringence phenomenon may be envisaged, the increasing number of lenses and poor efficiency for the resin injection molding in the glass lens manufacturing become problematic in order to keep up with the recent high image quality.
For a similar polarization-independent splitting method, the aforementioned crosstalk is generated due to a manufacturing error of a half mirror.
Japanese Patent Application Laid-open No. 63-058315 discloses a scanning optical system including a laser light source device, a focusing optical system, and a deflector. This scanning optical system is organized to reflect and deflect a light beam incident to the focusing optical system with a certain angle with respect to the optical axis of the focusing optical system by the deflector on a plane including an optical axis of the focusing optical system and a rotational axis of the deflector and scan a scanning target medium. In addition, the scanning optical system includes a light blocking means on an optical axis between the focusing optical system and the scanning target medium.
Japanese Patent Application Laid-open No. 6-003609 discloses a scanning optical device including a first optical system for modulating a light beam output from a light source, a deflection element for performing scanning in a deflective manner with the light beam output from the first optical system, and a second optical system for focusing the light beam onto the scanning target surface in a spot shape. In this scanning optical device, the part of or the entirety of the second optical system is configured as a lens molded using plastic or glass, The scanning optical device includes a light blocking member, which has a function of blocking the light beams passing through areas other than the optically effective area of the second optical system, between the deflection element and the second optical system.
Japanese Patent Application Laid-open No. 8-334719 discloses a scanning optical system including a polygonal mirror for deflecting a light beam, a housing hermetically enclosing the polygonal mirror, and a laser light source for emitting a laser light beam through a transparent parallel flat plate of the housing. In this scanning optical system, the parallel flat plate of the housing is parallel to a reflection surface of the polygonal mirror, and the laser light beam is incident in a direction inclined toward a sub-scanning direction with respect to a direction perpendicular to the reflection surface of the polygonal mirror.
Japanese Patent Application Laid-open No. 2001-305456 discloses a light scanning optical device including a light source means, a first optical system for inputting the light beam output from the light source means to a deflection means, and a second optical system for focusing the light beam deflected and reflected by the deflection means onto the scanning target surface. In this light scanning optical system, the second optical system includes at least a lens having a positioning unit in the center of the longitudinal direction of the lens and a bonding seat that does not make contact with the lens on the housing, so that the lens is fixed to the housing with use of a method of charging an adhesive into the gap between the bonding seat and the lens.
Japanese Patent Application Laid-open No. 2001-305457 discloses a light scanning optical device including a light source means, a first optical system for inputting a light beam output from the light source means to a polygonal mirror having a plurality of deflection surfaces, and a second optical system for focusing the light beam deflected and reflected by the polygonal mirror onto the effective scanning area on the scanning target surface. In this light scanning optical device, the light source means is always turned on even when the light beam scans areas other than the effective scanning area, and the polygonal mirror is provided such that a border portion between neighboring deflection surfaces is formed in an edge shape like a ridge line, and the width of the border portion spans about 1% or less of the width in the main-scanning direction of the light beam deflected and reflected by the polygonal mirror.
Japanese Patent Application Laid-open No. 2001-330790 discloses a light scanning optical device including a light source means, a first optical system for inputting the light beam output from the light source means to a deflection means, and a second optical system for focusing the light beam deflected and reflected by the deflection means onto an effective scanning area on the scanning target surface. In this light scanning optical device, the second optical system has one or more plastic lenses, and the plastic lens has a flange portion for reinforcing the end portion in the main-scanning direction, so that the corresponding area on an incidence plane and/or an emergent plane of the lens where the light beam incident to the flange portion transmits is formed in a non-mirror portion.
Japanese Patent No. 4409213 discloses a light beam scanning device in which light beams output from a light source portion are deflected at an equal angular velocity by a single deflection means, converted by a single optical means so as to scan each of a plurality of scanning targets at an equal velocity, and then, split by a splitting means so that each light beam is introduced to a plurality of scanning targets to carry out exposure scanning. In this light beam scanning device, the light source portion includes light emitting portions that generate the light beams and are at least equal in number to the number of the scanning targets. Meanwhile, a light blocking means for selectively transmitting or blocking the light beam directed to each scanning target is provided between the splitting means and each scanning target. An optical path is formed such that all of the light beams output from each light emitting portion are directed to all of the scanning targets. The splitting means is a unit for transmitting and reflecting each of the light beams so that the light beams may have the half strength, and a plurality of splitting means are provided between the deflection means and a plurality of light blocking means such that each of the light beams propagating through optical paths formed so as to correspond to scanning targets pass the optical paths the same number of times as each other, and the light beams that do not correspond to scanning targets, out of the light beams output from the respective light emitting portion, are blocked by the light blocking means.
However, the scanning optical systems disclosed in Japanese Patent Application Laid-open No. 63-058315 and Japanese Patent Application Laid-open No. 8-334719, the scanning optical device disclosed in Japanese Patent Application Laid-open No. 6-003609, and the light scanning optical devices disclosed in Japanese Patent Application Laid-open No. 2001-305456, Japanese Patent Application Laid-open No. 2001-305457, and Japanese Patent Application Laid-open No. 2001-330790 fail to take into consideration so-called ghost light included in the writing light beam modulated in response to the image information.
The light beam scanning device disclosed in Japanese Patent No. 4409213 is disadvantageous in terms of the cost because a lengthy member elongated in a main-scanning direction is necessarily used as the light blocking means. In addition, since it is necessary to arrange a plurality of splitting means over a plurality of stages in a direction perpendicular to the main-scanning direction, it is difficult to produce a thin optical scanning device. Furthermore, since four light beams are directed from the deflection means toward each scanning target on a plane perpendicular to the main-scanning direction while they are in parallel with each other, it is necessary to keep the interval of the light beams wide to some extent when it is necessary to selectively transmit or block the light beams with high accuracy with use of the light blocking means, but this leaves a concern of degradation in aberration. Moreover, this also requires highly accurate and precise installation of the light blocking means, thereby increasing the cost.